Vias are structures that are formed to enable electrical contact between different electrical interconnection layers in an integrated circuit. The via is typically formed in an insulating layer that is disposed between the two electrical interconnection layers, so that electrical contact only occurs at predetermined locations between the two electrical interconnection layers. The via is typically an etched hole that has a relatively high aspect ratio, meaning that it tends to be much deeper than it is wide. This is especially true as device geometries continually shrink.
Typically, a via is filled with a metallic electrical conduction system that makes contact between the underlying conduction layer disposed below the insulating layer in which the via is formed and the overlying conduction layer disposed above the insulating layer in which the via is formed. In a typical process, several different layers of metallic materials are used to fill the via. A thin titanium liner layer is first deposited as an adhesion layer and as a gettering layer. A thin titanium nitride liner layer is then deposited as a barrier layer to protect the underlying layers during subsequent processing. Finally, a tungsten plug is deposited to completely fill the via.
Because the via is a relatively small structure, it is important to reduce any electrical resistance created within the via by as great a degree as possible. One source of electrical resistance within the via is contact resistance between the metallic layers of the system as described above, such as can be caused by oxidation of the surfaces of the various layers. Oxidation of a layer tends to occur whenever the layer is exposed to the atmosphere, such as when it is transported from the chamber in which an underlying layer is deposited to the chamber in which an overlying layer is deposited. Oxidation is a particular problem with the titanium liner layer, which oxidizes very quickly in the presence of oxygen.
What is needed, therefore, is a system for reducing the contact resistance in a via by reducing the oxides that form on the titanium liner layer.